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<head><meta charset="utf-8"><title>prevent certain functions from calling other functions · general · Zulip Chat Archive</title></head>
<h2>Stream: <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/index.html">general</a></h2>
<h3>Topic: <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html">prevent certain functions from calling other functions</a></h3>

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<a name="207173013"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207173013" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Geoffrey Thomas <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207173013">(Aug 17 2020 at 17:36)</a>:</h4>
<p>Is there a general mechanism to annotate some function or block with X so that it/anything it calls cannot call a function annotated Y, and the compiler complains if you do this? My current use case is preventing calling blocking functions from critical sections in a kernel, but there's also things like not calling async-signal-unsafe functions in a signal handler, etc.</p>



<a name="207188501"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207188501" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Aaron Hill <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207188501">(Aug 17 2020 at 19:44)</a>:</h4>
<p>The best way to do that would probably be to make the function <code>Y</code> take a special marker type as a parameter</p>



<a name="207188517"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207188517" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Aaron Hill <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207188517">(Aug 17 2020 at 19:44)</a>:</h4>
<p>which can only be constructed in the proper context</p>



<a name="207188607"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207188607" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Aaron Hill <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207188607">(Aug 17 2020 at 19:45)</a>:</h4>
<p>there's no automated way of doing this that woul handle transitive function calls (e.g. <code>X() -&gt; A() -&gt; Y()</code>)</p>



<a name="207190120"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207190120" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> simulacrum <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207190120">(Aug 17 2020 at 20:00)</a>:</h4>
<p>there's some call graph analysis stuff from the embedded folks, I think</p>



<a name="207190158"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207190158" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> simulacrum <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207190158">(Aug 17 2020 at 20:00)</a>:</h4>
<p>but you can't get 100% without a parameter or so, yeah</p>



<a name="207200143"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207200143" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Geoffrey Thomas <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207200143">(Aug 17 2020 at 21:39)</a>:</h4>
<p>sure, but the problem is you can always construct the marker and pass it around, e.g., nothing prevents you from doing</p>
<div class="codehilite"><pre><span></span><code>let marker = OkToBlock::new();
let guard = CriticalSection::new();
let data = load_from_disk(sector, &amp;marker);
</code></pre></div>


<p>I need to make sure that <code>marker</code> and <code>guard</code> _don't_ exist at the same time within the same flow of control / same thread</p>



<a name="207200407"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207200407" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Geoffrey Thomas <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207200407">(Aug 17 2020 at 21:42)</a>:</h4>
<p>is cargo-call-stack the thing I want to look at? (I'd be totally fine with a lint)</p>



<a name="207210538"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207210538" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Jake Goulding <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207210538">(Aug 17 2020 at 23:56)</a>:</h4>
<blockquote>
<p>you can always construct the marker</p>
</blockquote>
<p>That's not strictly true, is it?</p>



<a name="207210548"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207210548" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Jake Goulding <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207210548">(Aug 17 2020 at 23:57)</a>:</h4>
<p>you just do more types</p>



<a name="207210570"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207210570" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Jake Goulding <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207210570">(Aug 17 2020 at 23:57)</a>:</h4>
<p>even your example is very similar to a mutex and a condvar</p>



<a name="207210590"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207210590" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Jake Goulding <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207210590">(Aug 17 2020 at 23:57)</a>:</h4>
<div class="codehilite"><pre><span></span><code><span class="kd">let</span><span class="w"> </span><span class="n">marker</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">OkToBlock</span>::<span class="n">new</span><span class="p">();</span><span class="w"></span>
<span class="kd">let</span><span class="w"> </span><span class="n">guard</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">CriticalSection</span>::<span class="n">new</span><span class="p">(</span><span class="n">marker</span><span class="p">);</span><span class="w"> </span><span class="c1">// move it here</span>
<span class="kd">let</span><span class="w"> </span><span class="n">data</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">load_from_disk</span><span class="p">(</span><span class="n">sector</span><span class="p">,</span><span class="w"> </span><span class="o">&amp;</span><span class="n">marker</span><span class="p">);</span><span class="w"> </span><span class="c1">// Can&#39;t use it cause it&#39;s moved</span>
</code></pre></div>



<a name="207210824"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207210824" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> teor <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207210824">(Aug 18 2020 at 00:01)</a>:</h4>
<p>What stops you from creating another marker at compile-time?</p>



<a name="207210922"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207210922" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Jonas Schievink  [he/him] <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207210922">(Aug 18 2020 at 00:03)</a>:</h4>
<p>Privacy, presumably?</p>



<a name="207210941"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207210941" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Jake Goulding <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207210941">(Aug 18 2020 at 00:03)</a>:</h4>
<p>depends how much control you want to have. ultimately you can <code>transmute</code> something to get a marker</p>



<a name="207210945"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207210945" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Jake Goulding <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207210945">(Aug 18 2020 at 00:03)</a>:</h4>
<p>but if you do something like</p>



<a name="207210975"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207210975" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Jonas Schievink  [he/him] <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207210975">(Aug 18 2020 at 00:03)</a>:</h4>
<p>You can make <code>transmute</code>s UB if you really want to</p>



<a name="207211164"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207211164" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Jake Goulding <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207211164">(Aug 18 2020 at 00:06)</a>:</h4>
<div class="codehilite"><pre><span></span><code><span class="k">mod</span> <span class="nn">locked</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w">    </span><span class="k">pub</span><span class="w"> </span><span class="k">struct</span> <span class="nc">Marker</span><span class="p">(());</span><span class="w"></span>

<span class="w">    </span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">start_thread</span><span class="p">(</span><span class="n">f</span>: <span class="nc">impl</span><span class="w"> </span><span class="nb">FnOnce</span><span class="p">(</span><span class="n">Marker</span><span class="p">))</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w">        </span><span class="n">f</span><span class="p">(</span><span class="n">Marker</span><span class="p">(()))</span><span class="w"></span>
<span class="w">    </span><span class="p">}</span><span class="w"></span>

<span class="w">    </span><span class="k">pub</span><span class="w"> </span><span class="k">struct</span> <span class="nc">CriticalSection</span><span class="p">(</span><span class="n">Marker</span><span class="p">);</span><span class="w"></span>

<span class="w">    </span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">exchange_for_critical_section</span><span class="p">(</span><span class="n">m</span>: <span class="nc">Marker</span><span class="p">)</span><span class="w"> </span>-&gt; <span class="nc">CriticalSection</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w">        </span><span class="n">CriticalSection</span><span class="p">(</span><span class="n">m</span><span class="p">)</span><span class="w"></span>
<span class="w">    </span><span class="p">}</span><span class="w"></span>

<span class="w">    </span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">exchange_for_marker</span><span class="p">(</span><span class="n">c</span>: <span class="nc">CriticalSection</span><span class="p">)</span><span class="w"> </span>-&gt; <span class="nc">Marker</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w">        </span><span class="n">c</span><span class="p">.</span><span class="mi">0</span><span class="w"></span>
<span class="w">    </span><span class="p">}</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>

<span class="k">fn</span> <span class="nf">main</span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w">    </span><span class="n">locked</span>::<span class="n">start_thread</span><span class="p">(</span><span class="o">|</span><span class="n">marker</span><span class="o">|</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w">        </span><span class="kd">let</span><span class="w"> </span><span class="n">cs</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">locked</span>::<span class="n">exchange_for_critical_section</span><span class="p">(</span><span class="n">marker</span><span class="p">);</span><span class="w"></span>
<span class="w">        </span><span class="kd">let</span><span class="w"> </span><span class="n">marker</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">locked</span>::<span class="n">exchange_for_marker</span><span class="p">(</span><span class="n">cs</span><span class="p">);</span><span class="w"></span>
<span class="w">    </span><span class="p">})</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
</code></pre></div>



<a name="207211209"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207211209" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Jake Goulding <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207211209">(Aug 18 2020 at 00:07)</a>:</h4>
<p><span class="user-mention" data-user-id="211727">@Jonas Schievink</span> what's that technique?</p>



<a name="207211222"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207211222" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Joshua Nelson <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207211222">(Aug 18 2020 at 00:07)</a>:</h4>
<p>what stops you from calling <code>start_thread</code> again inside the critical section?</p>



<a name="207211273"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207211273" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Jonas Schievink  [he/him] <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207211273">(Aug 18 2020 at 00:08)</a>:</h4>
<p>Just put a non-ZST inside without setting a repr, then all transmutes are UB</p>



<a name="207211295"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207211295" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Jake Goulding <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207211295">(Aug 18 2020 at 00:09)</a>:</h4>
<p>I was assuming that <code>start_thread</code> would actually start another thread and you'd want one marker per thread. You need to ensure that they cant be used across threads.</p>



<a name="207211314"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207211314" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Jonas Schievink  [he/him] <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207211314">(Aug 18 2020 at 00:09)</a>:</h4>
<p>We use a similar approach to the code above for embedded critical sections, just with a lifetime-constrained <code>CriticalSection</code>/<code>Marker</code> so you can't move them out</p>



<a name="207211327"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207211327" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Jake Goulding <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207211327">(Aug 18 2020 at 00:09)</a>:</h4>
<p>Yep ^</p>



<a name="207211386"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207211386" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Jake Goulding <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207211386">(Aug 18 2020 at 00:10)</a>:</h4>
<p>Which is the same technique that scoped threads use, right?</p>



<a name="207211390"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207211390" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Jonas Schievink  [he/him] <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207211390">(Aug 18 2020 at 00:10)</a>:</h4>
<p>Yeah</p>



<a name="207211767"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207211767" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Geoffrey Thomas <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207211767">(Aug 18 2020 at 00:16)</a>:</h4>
<p>(I'm fine with the restriction that only safe code gets the benefit of this checks, if you start transmuting stuff you're on your own.)</p>



<a name="207211805"></a>
<h4><a href="https://rust-lang.zulipchat.com#narrow/stream/122651-general/topic/prevent%20certain%20functions%20from%20calling%20other%20functions/near/207211805" class="zl"><img src="https://rust-lang.github.io/zulip_archive/assets/img/zulip.svg" alt="view this post on Zulip" style="width:20px;height:20px;"></a> Geoffrey Thomas <a href="https://rust-lang.github.io/zulip_archive/stream/122651-general/topic/prevent.20certain.20functions.20from.20calling.20other.20functions.html#207211805">(Aug 18 2020 at 00:17)</a>:</h4>
<p>is the idea here that you would thread a Marker through literally every function call that might need to either block or enter a critical section? that... seems... rough but maybe workable? I guess you wouldn't need to pass it into functions that do neither</p>



<hr><p>Last updated: Aug 07 2021 at 22:04 UTC</p>
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